1. Field of the Invention
The invention relates generally to downhole formation analysis and in particular to the compensation of measurements made by a magnetometer during measurement while drilling.
2. Summary of the Related Art
Oil well logging has been known for many years and provides information about various characteristics of the particular earth formation being drilled. In conventional oil well logging, after a well or borehole has been drilled, a probe known as a sonde is lowered into the borehole to determine characteristics of the formations which the well has traversed. The probe is typically a hermetically sealed cylindrical body which hangs at the end of a long cable which gives mechanical support to the sonde and provides power to various sensors and other instrumentation inside the sonde. The cable also provides communication channels for transferring information between the sonde and surface equipment. The sonde is typically pulled while the various sensors in the sonde take measurements, which are processed to determine the various characteristics of the formation. It thus becomes possible to measure parameters or characteristics of the earth's formations as a function of depth, that is, while the sonde is being pulled uphole. Although, such “wireline” measurements are normally made in real time, these measurements, however, are taken long after the actual drilling of the well.
There are also several teachings in prior art for taking measurements of formation parameters while the borehole is being drilled, i.e. while drilling the wellbore. For example, U.S. Pat. No. 5,397,893 to Minette, discloses a method for analyzing data from a measurement-while-drilling (MWD) formation evaluation logging tool which compensates for the rotation of the logging tool (along with the rest of the drillstring) during measurement periods. U.S. Pat. No. 5,513,528 to Holenka et al. (the “'528 patent”) discloses a method and apparatus for measuring formation characteristics as a function of azimuth about the borehole. The measurement apparatus of the '528 patent includes a logging while drilling tool which rotates in the borehole during drilling of the wellbores. In the '528 patent tool, the down vector of the tool is derived first by determining an angle “N” between a vector to the earth's north magnetic pole, as referenced to the cross sectional plane of the measuring while drilling (MWD) tool and a gravity down vector as referenced in the plane. The tool includes magnetometers and accelerometers placed orthogonally in a cross-sectional plane. Using the magnetometers and/or accelerometer measurements, the toolface angle can be determined. The angle N is transmitted to the tool, thereby allowing a continuous determination of the gravity down position in the logging while drilling tool
Neither Minette nor Holenka address possible sources of error in relying on readings made using magnetometers on a rotating drillstring. There are prior art methods that address the problem of correction of errors caused by metallic drill collars, casing, and accumulated debris. For example, U.S. patent application Ser. No. 11/256,794 of Blanz addresses the affect on magnetometer readings due to the rotation of the drillstring. U.S. Pat. No. 5,623,407 to Brooks discloses correction of bias errors due to interference and other sources by choosing the biases that minimize the variance of magnetic field magnitude at several sensor orientations. U.S. Pat. No. 6,966,211 to Wu discloses a method to correct errors in bias, scale-factor, misalignment of cross-axial magnetometers, and bias or scale-factor of axial magnetometer by requiring the magnitude of measured cross-axial magnetic field to be as constant as possible over several tool face angles at a survey point in a wellbore and the magnitude of the measured total magnetic field and dip angle equal to the reference values, respectively.
In general, the correction methods described above work reasonably well when the magnetometer is at a sufficiently large distance from magnetic disturbances on the downhole assembly. However, when a nuclear magnetic resonance (NMR) sensor is used in the tool, there is a major disturbance to the earth's magnetic field due to the magnets that are used for making the measurements. Typically, the magnets used in the NMR sensor are strong permanent magnets. For example, U.S. Pat. No. 6,111,408 to Blades discusses NMR measurements made utilizing permanent magnets with a field of 47 mT (470 Gauss). In contrast, the earth's magnetic field is of the order of half a Gauss. Even though the field of the magnets used for NMR measurements decays with distance, the disturbance to the earth's magnetic field can be significant to adversely affect the magnetometer measurements. The present invention provides apparatus and method that addresses this problem.